Artificial oxygen carriers (AOCs) are synthetic solutions having the ability to bind, transport and unload oxygen in the body. These synthetic solutions lack blood components such as immune cells and coagulation factors, and thus are called red blood cell (RBC) substitutes rather than blood substitutes.
Two types of artificial oxygen carriers are currently being developed: hemoglobin-based oxygen carriers (HBOCs), and synthetic red blood cells (sRBCs).
In order to avoid the spontaneous breakdown and toxicity of hemoglobin directly taken from red blood cells, purified human, animal or recombinant hemoglobin is used as raw materials in preparation of hemoglobin-based oxygen carriers. In addition, pure hemoglobin isolated from red blood cells has a disadvantage in that oxygen is not exactly delivered. To overcome this disadvantage, cross-linking, polymerization, encapsulation and the like should be performed during the preparation process, thus making the preparation process complex and increasing the production cost.
Hemoglobin-based oxygen carriers were approved by the US FDA for use in vertebrate animals, are in clinical phase III for use in humans, and were approved as pharmaceuticals by several countries in South Africa. HBOCs are known to have excellent in vivo stability.
Synthetic red blood cells are synthetic factors that mimic the function of red blood cells. Synthetic red blood cells transport oxygen, effectively deliver therapeutic drugs, and also enhance resolution in image diagnosis due to their ability to transport well-dispersed contrast agents while controlling the release rate. The particle size or oxygen capacity of these synthetic red blood cells is controlled by the amount of materials used in synthesis, or the configuration or structure of the components.
Among synthetic red blood cells, perfluorocarbon (PFC)-based synthetic red blood cells can be heat-treated, are effective for oxygen delivery and carbon dioxide removal, and have an excellent ability to diffuse in blood because they have a size equal to 1/40 of red blood cells. However, these PFC-based synthetic red blood cells have a shortcoming in that they have poor stability because they are cleared in vivo within 48 hours.
In an attempt to improve the stability, U.S. Patent Publication No. 2012-164231 discloses synthetic red blood cells prepared by emulsifying perfluorooctyl bromide or perfluorodecalin with 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA) or an equivalent lipid with a density higher than that of red blood cells, passing the emulsion multiple times through an extrusion membrane to produce submicron structures, coating Ca2+ and PO42− on the submicron structures to have a thickness of 5-20 nm, and mixing the coated submicron structures with carboxyethylphosphonic acid (CEPA). Herein, the CEPA that is added in the finishing step carboxylates the coating layer and inhibits self-aggregation of the synthetic red blood cells. The perfluorocarbon-based synthetic red blood cells have disadvantages in that, because Ca2+ and PO42− are added at the same time in the process of coating with Ca2+ and PO42−, most of the added Ca2+ and PO42− form the ceramic crystal calcium phosphate without being attached to the submicron structures, and thus the yield of coating on the submicron structures is low, and the production process cannot always provide submicron structures having the same coating, and thus has low reproducibility.
Accordingly, the present inventors have made extensive efforts to overcome the problems of conventional synthetic red blood cells, and as a result, have found that, when Ca2+ and PO42− coating layers are uniformly and sequentially formed while controlling the thicknesses of the Ca2+ and PO42− coating layers on synthetic red blood cells based on perfluorooctyl bromide, the oxygen capacity and oxygen release rate of the synthetic red blood cells can be controlled and the synthetic red blood cells can be retrieved and reused, thereby completing the present invention.